Blast gauge wherein four pressure sensors are positioned in a tetrahedral configuration on the surface of a sphere

ABSTRACT

The blast gauge has a spherical fairing containing four pressure sensors whose positions on the surface of the sphere form the apices of a tetrahedron. The fairing may have a solid metal sphere with cavities adapted to house the pressure sensors in which the metal is aluminum or an alloy thereof or a foam filled spherical shell containing cavities adapted to house the pressure sensors. The spherical fairing is carried by a tubular support adapted to contain the signal leads from the sensors. In a preferred form the blast gauge includes computing means adapted to receive the signals from the pressure sensors and provide data showing the velocity and direction of any shock wave impinging upon the spherical fairing.

BACKGROUND TO THE INVENTION

I. Field of the Invention

This invention relates to blast gauges and particularly to anon-directional gauge for blast measurements.

II. Description of Known Art

Measurement of blast involves the insertion of an instrumented body intothe flow field. Gauges of this type are known which feature aconventional pressure transducer housed within a streamlined fairing.The fairing is designed to cause minimal perturbation of the blast fieldbeing measured. Such known gauges only function accurately when alignedin such a way that the incident blast wave traverses the face of thetransducer at grazing incidence. Such alignment requires advanceknowledge of the direction of travel of the blast wave.

When measurements of blast waves are made in enclosed surroundings, suchas rooms, both direct and reflected waves are produced which travel indifferent directions. A gauge may be set to measure one of the waves,but it will be misaligned with respect to most of the other waves thatarise. The measurement of the reflected waves is most important as theymay interact and comprise a stronger force than the initial wave. If areflected wave should arrive at right angles to the face of thetransducer in such a gauge, the perceived pressure could be more thandouble the true static value. It is clear that such directional blastgauges are of little value when blast measurements need to be made inenclosed surroundings.

SUMMARY OF THE INVENTION

The present invention provides a non-directional blast gauge which willrecord the amplitude of a blast wave regardless of its orientation. Thegauge is also capable of providing directional information for each wavereaching it.

According to the present invention there is provided a blast gaugecomprising a spherical fairing containing four pressure sensors whosepositions on the surface of the sphere form the apices of a tetrahedron.

The spherical fairing may be hollow or comprise a solid block withrecesses to accept the sensors. The fairing may be constructed fromwood, metal or a synthetic polymer or composite based on such a polymersuch as resin bonded glass fiber. The fairing may comprise a metal orsynthetic polymer shell containing a foam filling. In a preferred formthe fairing is fabricated from aluminum or an alloy thereof. The size ofthe spherical fairing should be small to avoid disrupting the blast wavepattern. However the minimum size of the fairing is also limited by theparticular timing method being used and the consequent need to allow areasonable time delay between the shock waves impinging on thetransducers contained in it. Accuracy is reduced if the fairing is toosmall. In practice the minimum size is determined by the size of thesensors which are contained within it. The signals from the sensors arecarried by conductors through cavities in the interior of the fairinginto a tubular support or sting. The support must have sufficientrigidity to withstand any incident blast; however its diameter must alsobe as low as possible so as not to perturb the blast waves passing it.

The pressure sensors may be standard sensors which are commerciallyavailable or formed using piezo-sensitive film. "Endevco" sensors andthose supplied by Entran Devices, both from the United States ofAmerica, are particularly effective. However other types may be used.

The signals from the pressure sensors are fed to signal processing meanswhere the amplitude of the incident pressure on each sensor and its timeof occurrence are stored. The stored information may be used to providedetails of the amplitude and direction of successive blast waves whichfall on the gauge following an initial event.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention be clearly understood it will now bedescribed with reference to the accompanying drawings in which:

FIGS. 1A, 1B and 1C show a plan and two side views of a blast gaugeaccording to the invention.

FIG. 2 is a cross-sectional view of the gauge shown in FIG. 1.

FIG. 3 is a schematic view of a calibration system.

FIG. 4 is a calibration graph showing the relationship between shocktravel around the gauge to time.

FIG. 5 is a diagrammatic representation of a shock wave impinging on agauge according to the invention.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

A blast gauge according to the invention (See FIGS. 1A, 1B and 1C.)comprises a spherical shell 1, containing four tetrahedrally locatedsensors 2, 3, 4 and 5 whose sensitive faces are located at the surfaceof the sphere 1. Sensor 2 is located on the top of the sphere 1 whilethe other sensors, 3, 4 and 5, are located in the lower hemisphere. Thebase of the sphere 1 carries a sting 6 which supports the sphere 1 andforms a conduit for the leads from the sensors.

The spherical shell (See FIG. 2.) is made up of two sections 10 and 11which fit together. The upper section 10 contains a tubular cavity 12which contains a pressure sensor 13 (the same feature as the featurenumbered 2 in FIG. 1) whose sensitive face is located at the surface andwhich cavity carries signal leads from the sensor. The lower section 11contains a tubular cavity 14 which contains a pressure sensor 15 (thesame feature as the feature numbered 3 in FIG. 1) and two othercavities, not shown, containing sensors. A threaded cavity 16 contains ascrew 17 which holds the two sections 10 and 11 together. The leads fromthe pressure sensors 13 and 15 pass through the cavities in which theyare located, together with those from a third and fourth pressuresensor, not shown in FIG. 2 (but which correspond to the featuresnumbered 4 and 5 in FIG. 1), and meet in a small chamber 18. The leadspass through chamber 18 into a tubular sting 19 (the same feature as thefeature numbered 6 in FIG. 1) which acts as a conduit for the leads anda support for the spherical shell.

The electrical signals from the sensors are amplified and recorded inknown manner. The signals are preferably converted to and stored indigital form so that they can be processed by computer to provideinformation relating to the velocity and angle of attack of any shockwave which passes across the sphere.

In a preferred form of the gauge the sphere had a diameter of 50 mm andwas drilled with four tubular cavities. Each cavity housed an "Endevco"pressure transducer type 8507A. These transducers are approximately 10mm in length and 2 mm in diameter. The sting had a diameter of 5 mm.

Calibration tests were carried out on the gauge by mounting it in alarge diameter shock tube. The testing rig (See FIG. 3.) consisted of ashock tube 21 containing an angular position plate 22 attached to amounting bracket 23 fitted within the tube 21. The plate 22 carries aseries of mounting holes 24. The gauge 25 is positioned within the tube21 by attachment of its sting 26 to rotary mount 27 retained by one ofthe mounting holes 24. A small space 28 below the bracket 23 containsthe signal leads 29. The sting 26 is prevented from rotation during useby a clamping block 30.

The testing rig allows the attitude of the spherical blast gauge to bevaried in pitch and roll with respect to the blast wave directed downthe shock tube.

A series of tests were made using shock waves having a triangular front.Between each run the attitude of the gauge was altered. The first signalfrom one sensor was used to trigger a transient recorder which provideda record of both amplitude and time for the output from each sensor.

The recorded signals allow the time interval between the arrival of thefront of the shock wave to pass each sensor to be determined. By the useof suitable computing methods it is possible to determine the velocityof the initial shock wave.

The values of time interval obtained can be converted into thecorresponding values of shock velocity and the three direction cosinesof the normal to the shock front. As the orientation of the gauge willbe known in any measurement situation, the direction cosines of the axesto each of the four transducers can be calculated.

A calibration graph (See FIG. 4.) shows the relationship between theshock wave travel around a spherical gauge according to the inventionwith respect to time. The vertical axis shows the angle θ in degrees andthe horizontal axis time. The angle θ is the angle subtended by thenormal to the shock front to the center of the sphere (See FIG. 5.) inwhich a shock front 31, traveling in a direction shown by the arrows, isshown impinging on a sphere 32 containing pressure sensors, not shown.

I claim:
 1. A blast gauge comprising a spherical fairing containing fourpressure sensors whose positions on the surface of the sphere form theapices of a tetrahedron.
 2. A blast gauge according to claim 1, in whichthe fairing comprises a solid metal sphere with cavities adapted tohouse the pressure sensors.
 3. A blast gauge according to claim 2, inwhich the metal is aluminum or an alloy thereof.
 4. A blast gaugeaccording to claim 1, in which the fairing comprises a foam filledspherical shell containing cavities adapted to house the pressuresensors.
 5. A blast gauge according to claim 4, in which the shell ismetal.
 6. A blast gauge according to claim 1, 2, 3, 4 or 5, in which thespherical fairing is carried by a tubular support adapted to containsignal leads from the sensors.
 7. A blast gauge according to claim 6,which includes computing means adapted to receive signals from thepressure sensors and provide data showing velocity and direction of anyshock wave impinging upon the spherical fairing.
 8. A blast gaugeaccording to claim 1, 2, 3, 4 or 5, which includes computing meansadapted to receive signals from the pressure sensors and provide datashowing velocity and direction of any shock wave impinging upon thespherical fairing.